1. Field of Invention
This invention relates to a process for the production of polybenzimidazole membranes. More particularly, the invention relates to an improved process for the production of polybenzimidazole membranes that can be utilized in ultrafiltration.
2. Prior Art
Semipermeable membranes proposed in the past have been formed from a variety of materials, and are characterized by the ability to permit one component (e.g., ions or molecules) of a solution to pass through the membrane to the substantial exclusion of other components (e.g., other ions or molecules). Examples of substances heretofore recognized to possess this property include cellophane (i.e., regenerated cellulose), cellulose esters (e.g., cellulose acetate, cellulose butyrate, etc.), animal or protein membranes, polyelectrolyte complexes, ethyl cellulose, cross-linked polyacrylates, etc.
The semipermeable membranes of the prior art are of limited applicability in many separatory processes, such as reverse osmosis or ultrafiltration, because of inherent disadvantages relating to their chemical stability, strength, thermal stability, efficiency, length of life, and cost. Generally, the prior art membranes exhibit low thermal stability and, therefore, cannot be used successfully under conditions wherein the liquid undergoing treatment is at an elevated temperature. This may be a decided disadvantage in situations where the components to be separated only exist in solution at higher temperatures, or when it is economically advantageous to separate components of a solution at elevated temperatures rather than going through the expense of cooling. Furthermore, some membranes exhibit a decided decrease in efficiency upon an increase in temperature or pressure thereby limiting their range of applicability. Solvent susceptibility may be another factor affecting the applicability of a particular porous membrane to a separation process. Additionally, semipermeable membranes may be inappropriate for a particular application due to low solute rejection values or low flux.
Other factors which render the semipermeable membranes of the prior art of limited usefulness include their limited strength and chemical resistance and their extremely short operating lives at high pressures and temperatures. Low strength properties have generally been manifest in the form of the inability of the prior art films to operate at pressures in excess of about 1,000 p.s.i. or to operate at lower pressures for extended periods of time, especially at temperatures in excess of about 50.degree. C. Furthermore, in general commercial usage it is necessary that membranes be strong enough to withstand shipment, storage and general rough handling. Thus, the continued need to replace the prior art membranes due to mechanical failures has greatly limited their commercial usefulness.
In an attempt to overcome certain of the above-noted disadvantages of prior art membranes, membranes comprised of polybenzimidazole polymers have been provided. See, for example, U.S. Pat. Nos. 3,699,038, 3,720,607, 3,737,042, 3,841,492, 3,851,025, 4,020,142, 4,448,687 and 4,512,894. Such membranes, however, are fragile and contain many voids or finger-like incursions in the membrane structure that reduce their strength and plurality.
In the prior art, the polybenzimidazole membranes were coagulated in a non-solvent for the polybenzimidazole polymer. The non-solvent was preferably aqueous in nature and most preferably water.
Accordingly, it is an object of this invention to provide an improved process for the production of semipermeable polybenzimidazole membranes that can be utilized in ultrafiltration.
It is also an object of the invention to provide a process for the production of polybenzimidazole membranes which exhibit improved mechanical properties.
It is a further object of the invention to provide improved semipermeable membranes possessing chemical and thermal stability.
These and other objectives are obtained by preparing polybenzimidazole membranes by the process of the instant invention.